In a wind turbine generator system (wind farm) having a plurality of wind turbines, conventionally, there has been a demand from the grid side that reduces the output power to a predetermined value, in some cases. For example, U.S. Pat. No. 7,756,609 discloses a method in which, when an output-power reduction demand is issued, output power commands for the wind turbines are set in consideration of values obtained by proportionally dividing an output power reduction for the wind turbines and weighting values of the wind turbines.